Single-crystal zinc oxide and an electrophotographic plate made therefrom

ABSTRACT

AN IMPROVED ELECTROPHOTOGRAPHIC PLATE HAS A PHOTOCONDUCTIVE LAYER OF ONE OR MORE RELATIVELY LARGE WAFERS OF SINGLE-CRYSTAL, N-TYPE ZINC OXIDE. EACH WAFER IS FIXED TO, AND IN ELECTRICAL CONTACT WITH, AN ELECTRICALLY CONDUCTIVE BACKING. THE NOVEL METHOD OF CHARGING THE ELECTROPHOTOGRAPHIC PLATE COMPRISES (1) WETTING A MAJOR SURFACE OF THE ZINC OXIDE LAYER WITH A SOLUTION OF AN IONIZABLE SALT IN A RELATIVELY VOLATILE SOLVENT WHILE BIASING THE SOLUTION WITH A NEGATIVE VOLTAGE WITH RESPECT TO THE BACKING, AND (2) ALLOWING THE SOLVENT TO EVAPORATE, WHEREBY TO LEAVE AN ELECTROSTATIC CHARGE ON THE ZINC OXIDE SURFACE.

May 18, w M v 1 SINGLECRYSTAL ZINC OXIDE AND AN ELECTROPHOTOGRAPHIC PLATE MADE THEREFROM 1 Filed May 27. 1968 jmmv ran If TORNEY United States Patent O 3,579,332 SINGLE-CRYSTAL ZINC OXIDE AND AN ELEC- TROPHOTOGRAPHIC PLATE MADE THEREFROM Richard Williams, Princeton, NJ., assignor to RCA Corporation Filed May 27, 1968, Ser. No. 732,326

Int. Cl. G03g /00 US. Cl. 96--1.8 6 Claims ABSTRACT OF THE DISCLOSURE An improved electrophotographic plate has a photoconductive layer of one or more relatively large wafers of single-crystal, n-type zinc oxide. Each wafer is fixed to, and in electrical contact with, an electrically conductive backing. The novel method of charging the electrophotographic plate comprises (1) wetting a major surface of the zinc oxide layer with a solution of an ionizable salt in a relatively volatile solvent while biasing the solution with a negative voltage with respect to the backing, and (2) allowing the solvent to evaporate, whereby to leave an electrostatic charge on the zinc oxide surface.

BACKGROUND OF THE DIVENTION This invention relates generally to an electrophotographic plate and a method of charging it electrostatically. More particularly, the invention relates to an improved electrophotographic plate, having a photoconductive layer of single-crystal zinc oxide, and to a novel method of applying an electrostatic charge to the improved electrophotographic plate. The improved electrophotographic plate and novel method of charging it are particularly useful for providing means for copying images that are either to be subsequently erased or transferred to a transfer sheet.

It has been proposed to use photoconductive layers of zinc oxide on recording elements, such as electrophotographic plates, but such prior-art photoconductive layers usually comprised zinc oxide powder or minute crystals dispersed in an insulating binder, such as a silicone resin, for example. While such prior-art photoconductive layers were quite suitable for use in most electrophotographic processes, the surfaces of such photoconductive layers were usually not smooth enough to provide a maximum of image resolution.

Also, while prior-art photoconductive plates can be electrostatically charged adequately by conventional corona discharge devices, they do not lend themselves easily to charging by means of an electrically biased solution of an ionizable salt in a volatile solvent, as employed in the novel method, because such prior-art photoconductive layers may have minute pin holes therein, thereby providing a current-leakage path for the resultant charge to the conductive backing.

SUMMARY OF THE INVENTION Briefly, the improved electrophotographic plate comprises a photoconductive layer of one or more relatively large Wafers of single-crystal zinc oxide on a relatively electrically conductive backing. The novel method of charging a recording element, such as the improved electrophotographic plate, comprises (1) wetting a major surface of the photoconductive layer with a solution of an ionizable salt in a relatively volatile solvent while biasing the solution with a voltage with respect to the conductive backing of the plate, and (2) allowing the solvent to evaporate from the surface, whereby to leave an electrostatic charge thereon.

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The improved electrophotographic plate has advantages over prior-art electrophotographic recording elements in that the improved electrophotographic plate requires less dark adaptation before use, and has a higher resistivity, a lower charge decay rate, and a higher image resolving power.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of one embodiment of an improved electrophotographic plate, employing a single wafer of single-crystal zinc oxide;

FIG. 2 is a plan view of another embodiment of an improved electrophotographic plate, employing a plurality of wafers of single-crystal zinc oxide;

FIG. 3 is a cross-sectional view of the electrophotographic plate shown in FIG. 2, taken along the line 3--3, and viewed in the direction indicated by the arrows;

FIG. 4 is a cross-sectional view, partly schematic, of a prior-art electrophotographic recording element and conventional apparatus for charging it;

FIG. 5 is a side view of the improved electrophotographic plate shown in FIG. 1 and apparatus for electrostatically charging it by the novel method of the present invention;

FIGS. 6 and 7 are side views of the improved electrophotographic plate and means for exposing and developing it, respectively; and

FIG. 8 is a side view, partly schematic, of the improved electrophotographic plate with a developed image thereon, and apparatus for transferring the image to a transfer sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawing, there is shown an improved electrophotographic plate 10, comprising a photoconductive layer 12 fixed to an electrically conductive backing 14 by electrically conductive binding means 16, such as solder or silver paste. The photoconductive layer 12 comprises a relatively large wafer 13 of singlecrystal, n-type, zinc oxide whose donor concentration is between about 10 cmr and 5 X 10 CIILT3.

The wafer 13 may have a thickness between about 10 mm. and 1 mm., and the area of the upper major surface 18 should be at least 0.25 cm. Thus, if the wafer 13 is square, each side should have a length of about 0.5 cm. It is desirable, however, that the upper major surface 18 of the photoconductive layer 12 should have as large an area as possible. Single wafers 13 of singlecrystal, n-type, zinc oxide have been grown to a size large enough to provide square Wafers whose sides are at least 5 cm. in length, providing an area of the major surface 18 of at least 25 cm.

The conductive backing 14 is preferably a sheet of metal, such as copper, aluminum, or stainless steel, for example, and may protrude beyond the edges of the wafer 13 to protect the latter, as shown. The lower major surface 20 of the wafer 13 is preferably coated with a very thin layer (not shown) of indium so as to provide a good electrical contact with the conductive backing 14, through the conductive binding means 16.

The major surface 18 of the photoconductive layer 12 should preferably be parallel to the cards of the singlecrystal zinc oxide because it has been found that, in this orientation, the major surface 18 may be polished with an HCl etch to a smoother surface than possible in other crystal orientations. Such a surface provides means for obtaining the highest resolution of an image to be developed subsequently thereon.

The electrophotographic plate 10 is superior to elective layers comprise finely divided zinc oxide in an insulating resin binder because the improved photographic plate 10 does not have to be dark adapted before use, as is usually necessary with most prior-art electro hotographic recording elements. Also, the improved electrophotographic plate 10 can retain thereon an electrostatic charge whose amplitude is at least an order of magnitude greater, and whose charge decay rate (in darkness) is also relatively much slower, than possible with prior-art recording elements.

The improved electrophotographic plate 10 retains a greater electrostatic charge per unit area than prior-art recording elements because a greater electric field can be produced at the surface 18 during the charging process (by any charging method). In the improved electrophotographic plate 10, the ZnO crystal Wafer 13 is conductive before charging, having a free carrier concentration in the order of 10 carriers per cm. as stated above. During charging, a Schottky barrier forms at the surface 18, creating a thin region of material having a thickness in the order of 0.5 l cm. which is much less than the total thickness of the crystal and is completely devoid of free carriers. This surface layer has the very high resistivity (10 ohm-cm.) characteristic of intrinsic zinc oxide. The potential difference applied by charging is entirely across this thin region. In this way, an applied potential difference of 100 volts produces an electric field at the surface 18 in the order of 2 l0 volts per cm. The total charge retained by the surface 18 is proportional to the surface field. In prior-art recording elements, by way of comparison, electrostatic charging usually produced a potential difference of about 500 volts across a region of thickness of about 25 l0- cm., providing a corresponding electric field of about only 2 1O volts per cm., and a correspondingly smaller charge retained on the surface.

Images developed on the major surface 18 of the photoconductive layer 12 have a greater resolution than those on prior-art recording elements because the major surface 18 can be provided with a smoother finish than is usually available, or possible with, on prior-art recording elements. The surface 18 can also be more easily erased and reused than surfaces on prior-art recording elements for the same reason.

Since the area of the major surface 18 of one singlecrystal wafer 13 limits the size of the electrophotographic plate 10, a plurality of wafers 13 can be arranged to provide a continuous image surface of a larger electro photographic plate. Referring now to FIGS. 2 and 3, there is shown a composite electrophotographic plate 22 comprising a plurality of square wafers 13 whose upper major surfaces 18 are disposed in substantially the same plane, forming a continuous surface. Each of the wafers 13 in the composite electrophotographic plate 22 are as close to each other as possible. If, however, any space should exist between adjacent wafers 13, the space may be filled in with an insulating resin, such as silicone resin, alkyd resin, or epoxy resin, for example.

Each of the wafers 13 of the composite electrophotographic plate 22 is fixed to, and in electrical contact with, a conductive backing 14a by electrically conductive binding means 16a in the same manner and by the same means described for the binding of the wafer 13 to the conductive backing 14 of the electrophotographic plate 10.

The electrophotographic plates and 22 may be electrostatically charged, in darkness, by any prior-art, conventional, charging means used for charging prior-art recording elements. For example, the electrophotographic plates 10 and 22 may be charged by a corona discharge device 24, illustrated in FIG. 4. In FIG. 4, a prior-art recording element 26, comprising a layer 28 of finely divided zinc oxide in a resin binder fixed to a relatively conducting paper backing 30, is disposed on a grounded metallic plate 32 for receiving a negative charge from the corona discharge device 24. A wire electrode 32 of the corona discharge device 24 is connected to the negative terminal of a unidirectional source 34 of voltage of at least 6,000 volts, and the negative terminal of the voltage source 34 is connected to the grounded plate 32. A shield 36 of the corona discharge device 24 is also grounded. Under these conditions, a corona discharge emanating from the wire electrode 32 produces negative ions in the air which are attracted to the surface of the recording element 26, providing a negative electrostatic charge thereon in a manner well known in the art. The improved electrophotographic plates 10 and 22 may be substituted for the recording element 26, in FIG. 4, and can also be charged by this prior-art method.

In accordance with the novel method of the present invention, the electrophotographic plates 10 and 22 can be charged with a relatively lower voltage, about 50 volts, for example, than is possible by the prior-art method illustrated in FIG. 4. Means for carrying out the improved method of charging the improved electrophotographic plate 10, for example, are shown in FIG. 5. Referring now to FIG. 5, there is shown a source 40 of unidirectional voltage, illustrated as a battery of about 50 volts, having its positive terminal connected to the grounded conductive backing 14 of the electrophotographic plate 10 and its negative terminal connected to a wetted, solution-retaining, sponge-like member 42.

The member 42 is Wetted or moistened with a solution of an ionizable salt, such as potassium chloride (KCl), tetramethyl ammonium chloride [(CH NCl], tetrabutyl ammonium iodide ([CH (CH NI, and (2-chloroethyl) trimethyl ammonium chloride in a relatively volatile solvent, such as ethyl formate (HCOOC H methyl acetate (CH COOCH and acetone (CH COCH for example. The concentration of the ionizable salt in the solvent is not critical, and saturated solutions of one of the salts in one of the organic solvents can be used.

The improved method of charging the electrophotographic plate 10 comprises (1) wetting the surface 18 by moving the member 42, moistened With a solution of one of the ionizable salts in one of the relatively volatile solvents, across the upper major surface 18 in the direction indicated by arrows 44 and 46, while the member 42 is biased negatively with respect to the conductive backing 14, as shown in FIG. 5, and (2) allowing the deposited solution to evaporate from the surface 18, leaving the surface 18 electrostatically charged negatively. Only a thin film of solution need be deposited on the surface 18 by the member 42 so that evaporation of the film can occur in a matter of a few seconds.

Although the biasing voltage is illustrated as a 50 volt source, it has been found that the biasing voltage may be in the range between a fraction of a volt to about 1000 volts, depending on the type of solution and recording element used. Regardless, however, of the biasing voltage required in a particular application of the novel charging method, the biasing voltage is less than necessary to produce a corona discharge by prior-art conventional charging methods.

A recording element requiring a positive charge, such as a recording element having a photoconductive surface of selenium thereon, for example, may also be charged by the improved method. To accomplish this, the solution is biased positively with respect to the conductive backing of the recording element.

After the recording element 10 has been charged negatively, it may be exposed to a light image, as, for example, the image of the arrow 48, in FIG. 6, through a suitable optical system 50 to discharge the previously charged photoconductive layer 12 and to form an electrostatic latent image thereon. The latent image may now be developed by any suitable electroscopic toner, either liquid or powder by a method known in the art to provide a developed image 52 on the surface 18, as shown in FIG. 7. The unfixed image 52 of electroscopic particles may now be transferred to a transfer sheet 54 by placing the transfer sheet 54 in contact with the electroscopic image 52 and providing a transfer voltage, as by a corona discharge device 56, as shown in FIG. 8. The thin wire electrode 58 of the corona discharge device 56 is connected to the positive terminal of a suitable unidirectional voltage source 58 and the negative terminal of the voltage source 58 is connected to the grounded conductive backing 14 of the electrophotographic plate 10.

Although the improved electrophotographic plates and 22 have been described as comprising wafers 13 of square shape, the wafers 13 may have any desired shape. For example, the wafers 13 in the electrophotographic plate 22 may be hexagonal in shape and still provide a continuous surface 18 for the entire plate. Also, the solutions described herein used for charging the electrophotograp'hic plates by the novel charging method are merely illustrative of ionizable salts in volatile solvents, and the examples given are not intended to be considered in a limiting sense.

What is claimed is: 1. A11 electrophotographic plate comprising a wafer of single-crystal, n-type, zinc oxide having a pair of opposite major surfaces, said wafer having a thickness of at least 10- mm.,

each of said major surfaces having an area of at least .25 sq. cm.,

said plate having a backing of electrically conductive material, and means binding one of said major surfaces of said wafer to said backing and in electrical contact therewith.

2. An electrophotographic plate as described in claim 1, wherein said Wafer of single-crystal, n-type, zinc oxide has a donor concentration in the range between 10 cm." and 5x10 cm.- and said thickness is in a range between 10 mm. and 1 3. An electrophotographic plate as described in claim 1, wherein,

said backing comprises a relatively rigid sheet of a metal, and

said binding means comprises a metallic layer on said one major surface and means bonding said metallic layer to said sheet. 4. An electrophotographic plate as described in claim 1, wherein said wafer of n-type zinc oxide has a thickness between about 102 mm. and 1 mm. said plate comprises a plurality of wafers substantially similar to said wafer, one major surface of each of said wafers is fixed to said backing, and the other major surface of each of said wafers defines a planar surface. 5. An electrophotographic plate as described in claim 1, wherein said plate comprises a plurality of wafers, each of said wafers being substantially similar in composition to said wafer of n-type zinc oxide, each of said wafers having one of its major surfaces fixed to said backing, adjacent wafers on said backing being contiguous to each other, and the other major surfaces of said wafers defining a substantially continuous surface. 6. An electrophotographic plate as described in claim 5, wherein an insulating resin is disposed between said adjacent wafers, whereby to provide a substantially continuous surface of said other major surfaces of said wafers.

References Cited UNITED STATES PATENTS 3/1968 Nail 96-48 OTHER REFERENCES GEORGE F. LESMES, Primary Examiner M. B. WIT'PENBERG, Assistant Examiner U.S. Cl. X.R. 

